Inkjet recording apparatus

ABSTRACT

An inkjet recording apparatus including a recording head having a nozzle, a pressure chamber, and a pressure generation section; and a drive signal generator which generates a drive signal for applying at least one drive pulse, wherein the apparatus ejects the ink droplet from the nozzle by applying the drive signal to activate the pressure generation section, wherein the drive signal generator generates the drive signal within one pixel period in chronological order including a first expansion pulse to expand the volume of the pressure chamber, a contraction pulse, and a second expansion pulse, and wherein a contraction pulse width is 0.1 AL through 0.5 AL, where AL represents a half of an acoustic resonance period of the pressure chamber, and |Von|/|Voff| is 1.3 through 10, where Von and Voff respectively represent drive voltages of the first expansion pulse and the contraction pulse.

TECHNICAL FIELD

This invention relates to inkjet recording apparatus for ejecting an inkdroplet (liquid droplet) from a nozzle.

BACKGROUND

In an inkjet apparatus, in order to realize a high quality recording,the ink dot diameter needs to be made small. As a method of reducing therecording dot diameter, it is conventionally known to utilize a“pull-push driving” system where a pressure chamber communicating to anozzle opening is contracted after temporarily expanded. According tothis system, the mass of each ink droplet can be reduced, and therecording dot diameter can be minified.

As the recording heads utilizing piezoelectric elements as pressuregeneration devices, there are a system of applying a vibration plate(for example, a laminated piezoelectric layer method and a deflectionmode method), and a shear deformation system where a partition wall of apressure chamber is shear deformed without using the vibration plate.

In the laminated piezoelectric layer method which changes the volume ofthe pressure chamber via the vibration plate, since the piezoelectricelement is disposed outside the pressure chamber, the shape and size ofthe piezoelectric element is not so much restricted, and it is possibleto generate high pressure by using a powerful piezoelectric element,thus this method is good at ejection capability and ejection control ofthe ink droplet. However, the structure of such an inkjet head becomescomplicated, manufacturing of a large capacity head is difficult, and ahead having about 100 channels may be a limit.

In contrast, since the head of shear deformation mode system has asimple structure where grooves are formed to be pressure chambers in apiezoelectric element, a large capacity head having several hundredchannels is possible to be manufactured. However, especially in thecases where drive signals of a rectangular pressure wave are applied tothe recording head of shear mode system, ejection of a minute droplet isdifficult due to the influence of pressure wave vibration in thepressure chamber.

In Examined Japanese Patent Application Publication No. 4161631 (PatentDocument 1) described is a method of forming a minute droplet byutilizing a head of the shear mode system, applying voltages to deformthe pressure chamber in order of a first expansion, contraction and asecond expansion, and by controlling a ratio of the voltages and a widthof the contraction pulse. Wherein, a pulse width of the first expansionpulse is referred as t1, a pulse width of the contraction pulse as t2,and a pulse width of the second expansion pulse is referred as t3.

However in a case where the pressure chamber is driven with acontraction pulse width t2 as described in the above mentioned PatentDocument 1, a pressure wave vibration which is generated at the edgeportion of the drive pulse cannot be effectively canceled and residualvibration remains largely. Therefore, to execute high frequency drive inthis state is difficult. Further, Patent Document 1 describes an exampleof applying a second contraction pulse is applied to cancel the residualvibration. However, by applying the second contraction pulse, the totalwaveform of the pulses becomes long, which leads to decrease of thedrive frequency. Further, even in the case where t2+t3=AL (AL: half ofthe acoustic resonance period of the pressure chamber) is satisfiedwithout applying the second contraction pulse, as described in PatentDocument 1, the residual vibration cannot be sufficiently canceled,which leads to greatly decreasing the drive stability. In order toobtain sufficient drive stability, it is necessary to wait for asufficient time period until the residual vibration decays before thenext drive, which results in the decrease of drive frequency.

Further, according to Patent Document 1, the droplet volume may bereduced to be 10 pl, however further reduction of the droplet volume isrequired in market.

SUMMARY

Accordingly, an objective of the present invention is to provide a drivemethod of inkjet head which is capable of stably ejecting a furtherminified droplet with a high drive frequency.

Embodiments of inkjet recording apparatus reflecting an aspect of thepresent invention are:

(1) An inkjet recording apparatus including: a recording head having anozzle to eject an ink droplet, a pressure chamber connected to thenozzle, and a pressure generation section to vary a volume of thepressure chamber; and a drive signal generator which generates a drivesignal for applying at least one drive pulse to eject the ink droplet,wherein the inkjet recording apparatus is configured to eject the inkdroplet from the nozzle by applying the drive signal to activate thepressure generation section, wherein the drive signal generator isconfigured to generate the drive signal within one pixel period inchronological order including a first expansion pulse to expand thevolume of the pressure chamber, a contraction pulse to contract thevolume of the pressure chamber, and a second expansion pulse to expandthe volume of the pressure chamber again, and wherein a contractionpulse width is not less than 0.1 AL and not more than 0.5 AL, where ALrepresents a half of an acoustic resonance period of the pressurechamber, and |Von|/|Voff| is not less than 1.3 and not more than 10,where Von represents a drive voltage of the first expansion pulse, andVoff represents a drive voltage of the contraction pulse.

(2) The inkjet recording apparatus described in (1), wherein a pulsewidth of the second expansion pulse is not less than 0.2 AL and not morethan 0.6 AL.

(3) The inkjet recording apparatus described in (1) or (2), wherein asum of pulse widths of the contraction pulse and the second expansionpulse is not less than 0.3 AL and not more than 0.9 AL.

(4) The inkjet recording apparatus described in any one of (1) to (3),wherein pulse width of the first expansion pulse is 1 AL.

(5) The inkjet recording apparatus described in any one of (1) to (4),wherein a pulse width of the contraction pulse is less than a pulsewidth of the second expansion pulse.

(6) The inkjet recording apparatus described in any one of (1) to (5),wherein the apparatus varies a pulse width of the contraction pulsewithin a range of 0.1 AL through 0.5 AL to control a volume of the inkdroplet.

(7) The inkjet recording apparatus described in any one of (1) to (6),wherein the drive signal generator is configured to generate the drivesignal for applying a plurality of drive pulses within one pixel period,in such a manner that a plurality of ink droplets each ejected by eachof the plurality of drive pulses are united before or after landing onrecording medium to form a single pixel.

(8) The inkjet recording apparatus described in (7), wherein theplurality of drive pulses include plural types of drive pulsesrespectively having contraction pulse widths different with each otherwithin a range from 0.1 AL through 0.5 AL, and the respective pluraltypes of drive pulses cause to eject plural types of ink droplets havingdifferent volumes with each other.

(9) The inkjet recording apparatus described in any one of (1) to (8),wherein a drive voltage of the second expansion pulse is equal to adrive voltage Von of the first expansion pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of a line typeinkjet recording apparatus;

FIG. 2 is a schematic diagram showing a configuration of an inkjet headunit;

FIG. 3 is diagram showing an example of arrangement for inkjet head inthe inkjet head unit;

FIG. 4 a is a partially sectional perspective diagram of an inkjet headchip for three-cycle drive system;

FIG. 4 b is a sectional view of the ink channel viewed from thedirection of channel arrangement for three-cycle drive system;

FIGS. 5 a-5 c are diagrams showing movements of the inkjet head at thetime of ink ejection in three-cycle chive system;

FIGS. 6 a-6 c are diagrams of time sharing movements of the inkjet headat the time of ink ejection in three-cycle drive system;

FIG. 7 a is a partially sectional perspective diagram of an inkjet headchip in independent drive system;

FIG. 7 b is a sectional view of the ink channel viewed from thedirection of channel arrangement for independent drive system;

FIGS. 8 a-8 c are diagrams showing movements of the inkjet head at thetime of ink ejection in independent drive system;

FIG. 9 shows a drive signal according to Patent Document 1;

FIG. 10 shows a drive signal of the present invention to apply a singledrive pulse in one pixel period to eject an ink droplet;

FIG. 11 shows a drive signal of the present invention to apply pluraldrive pulses in one pixel period respectively to eject each ink droplet;

FIG. 12 is a time chart for drive signals applied to an electrode ofeach pressure chamber in each group of A, B, and C.

FIG. 13 shows relationships between a droplet volume and contractionpulse width;

FIG. 14 shows relationships between maximum stable ejection velocity andsecond contraction pulse width;

FIG. 15 shows relationships between a droplet volume and drive voltageratio;

FIG. 16 shows a pressure wave of the present invention;

FIG. 17 shows a pressure wave of the comparative example;

FIG. 18 shows a pressure wave of the comparative example;

FIG. 19 shows maximum stable velocity of droplet ejection;

FIG. 20 shows relationships between a droplet volume and contractionpulse width; and

FIG. 21 shows relationships between a droplet volume and contractionpulse width.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described below in detail however thepresent invention is not limited by the description below.

FIG. 1 is a schematic drawing showing the configuration of the line typeinkjet recording apparatus I.

As shown in FIG. 1, elongated rolled recording medium 10 is pulled-outand conveyed from rolling-out roll 10A in a direction of arrow X byunillustrated drive means.

Elongated rolled recording medium 10 is conveyed while being trained andsupported by back roll 20. From inkjet head unit 30, ink is ejectedtoward recording medium 10, to pedal in image formation based on imagedata. Inkjet head unit 30 is provided with a plurality of recordingheads 31 corresponding to an ejection width in the width direction ofthe recording medium.

FIG. 2 shows an example for arrangement of inkjet head 31 in inkjet headunit 30. In this example, all inkjet head 31 are arranged in positionsof a same height with respect to intermediate tank 40 temporarilyreserving the ink. Since an ejection width of each inkjet head is lessthan the outer shape width size of the recording head, a plurality ofinkjet heads are arranged in zigzag with respect to the conveyingdirection of the recording medium. In the example shown in FIG. 2, theplurality of inkjet heads, each corresponding to the ejection width inthe width direction of recording head, are arranged in two rows zigzagarrangement.

FIG. 3 is a diagram showing a relationship of outer shape, ejectionwidth and a zigzag arrangement of inkjet head 31. Since the number ofinkjet heads 31 and the number of rows in zigzag arrangement areproperly determined according to the ejection width and the like, thearrangement is not limited to that shown in FIG. 3.

In FIG. 1, the ink is supplied via plural ink tubes 43 to each inkjethead 31 from intermediate tank 40 which adjusts a back-pressure of theink in inkjet head 31. In the present embodiment, ink tube 43 in FIG. 1represents a plurality of ink tubes.

Ink supply to intermediate tank 40 is conducted by liquid sending pump Pprovided between reservoir tank 50 to reserve ink and supply pipe 51.

Recording medium 10 on which an image has been formed is dried at dryingsection 100 and is rolled on take-up roll 10B.

In a state that inkjet head 31 stands still, image recording is executedwhile the recording medium is conveyed in the conveyance direction.While the recording medium is being conveyed, drive signals are selectedbased on image data for each pixel period and ink ejection state changesaccordingly.

Each inkjet head 31 is arranged such that the nozzle plane is opposed toa recording surface of recording medium 10, and electrically connectedvia flexible cable 6 (FIG. 4) to drive signal generator 100 (refer toFIG. 5 a) or 101 (FIG. 8 a) for generating the drive signals.

FIG. 4 a is a partially sectional perspective diagram of a head chipportion of shear mode type inkjet head 31 for three-cycle drive system,and FIG. 4 b is a sectional view of the ink channel 28 viewed from thedirection of channel arrangement of shear mode type inkjet head 31 forthree-cycle drive system.

FIG. 7 a is a partially sectional perspective diagram of a head chipportion of shear mode type inkjet head 31 for independent drive system,and FIG. 7 b is a sectional view of the ink channel 28 viewed from thedirection of channel arrangement of shear mode type inkjet head 31 forindependent drive system.

In the figures, 310 represents the head chip, and 22 represents a nozzleforming member adhered on a front surface of head chip 310.

FIGS. 5 a-5 c are sectional diagrams of channel rows in shear mode typeinkjet head for three-cycle drive system viewed from an elongateddirection of channels in inkjet head.

FIGS. 8 a-8 c are sectional diagrams of channel rows in shear mode typeinkjet head for independent drive system viewed from an elongateddirection of channels in inkjet head.

In the present specification, a face of the head chip from where the inkis ejected is designated as a “front face”, and the opposite face as“back face”. And top and bottom outer faces of the head chip sandwichingthe channels arranged in parallel in the drawing are respectivelydesignated as a “top face” and a “bottom face”.

Head chip 310 is provided with channel rows where a plurality ofchannels 28 separated by separation wall 27 are arranged in parallel.Here, the channel rows have 512 pieces of channel 28, however the numberof channels is not restricted.

Each separation wall 27 in this case is configured with two sheets ofpiezoelectric materials 27 a and 27 b, however, it is applicable toarrange the piezoelectric material for at least a part of partition wall27.

There is no restriction to the piezoelectric material used in thepiezoelectric materials 27 a and 27 b, provided that deformation occurswhen voltage is applied. Publicly known materials can be used as thepiezoelectric material. It can be a substrate made of an organicmaterial. However, the substrate made of a piezoelectric non-metallicmaterial is preferably utilized. For example, the substrates made ofthis piezoelectric non-metallic material include a ceramic substrateformed by molding and burning, and a substrate formed by coating andlamination. The organic material includes an organic polymer, and ahybrid material of the organic polymer and inorganic substance.

The ceramic substrate includes PZT (PbZrO₃—PbTiO₃) and third componentadded PZT. The third component contains Pb(Mg_(1/3)Nb_(2/3))O₃,Pb(Mn_(1/3)Sb_(2/3))O₃, Pb(Co_(1/3)Nb_(2/3))O₃. Further, BaTiO₃, ZnO,LiNbO₃ and LiTaO₃ can also be used to produce it.

In cases where two sheets of piezoelectric material is adhered such thateach polarizing direction being opposite with each other as the presentembodiment, the amount of shear displacement becomes doubled compared tothe case of single sheet piezoelectric material, which causes anadvantage that at most a half drive voltage is required to obtain thesame amount of displacement.

In FIGS. 5 a-c, three ink channels (28A, 28B, and 28C) which being apart of ink channels 28 are shown. These ink channels are separated byseparation walls 27A, 27B, 27C and 27D.

In FIGS. 8 a-c, three ink channels (28A, 28B, and 28C) which being apart of ink channels 28 are shown. These ink channels are separated byair channels 128.

At a front surface and a rear surface of head chip 310, a front sideopening and a rear side opening of each ink channel 28 are respectivelyarranged to be opposed. Each ink channel 28 is configured as strait typewhere size and shape for the channel is almost unchanged in thelongitudinal direction from the front side opening through the rear sideopening.

One end of ink channel 28 (hereinafter, this may be called as nozzleend) is connected to nozzle 23 fowled on nozzle forming member 22, andthe other end (hereinafter, may be called as manifold end) is connectedto ink tube 43 via common ink room 47 and ink supply port 25.

As shown in FIGS. 4 a-b, FIGS. 5 a-c, FIGS. 7 a-b, and FIGS. 8 a-c, allinner surface of each channel 28 is adhered with electrode 29 formed ofmetal layer. Namely pieces of electrode 29 on the separation wallopposing each other in each channel are electrically connected.Electrode 29 in the ink channel is connected to drive signal generator100 or 101 via connecting electrode 300 and anisotropicelectro-conductive film 6.

Next, an example of manufacturing method of this kind of inkjet head 31will be described below. However the present invention is not restrictedto this.

Firstly, plate-shaped piezoelectric materials 27 a and 27 b configuredwith PZT polarized in thickness direction are laminated such that thedirections of polarization become different with each other, and adheredwith an epoxy-type adhesive agent. Further, on the surface of upperpiezoelectric material plate 27 a, a dry film is adhered.

Next, from the side of said dry film, plural parallel grooves aregrounded by the use of dicing blade and the like, which become channels28 and 128. Each of the grooves is formed from one end to the anotherend of piezoelectric materials 27 a and 27 b, and is ground with acertain depth D reaching to half of the under side piezoelectricmaterial 27 b (refer to FIG. 4 b), to be a strait type groove in whichsizes and shapes are almost unchanged in the longitudinal direction.

After that, from the groove formed side of the piezoelectric materials,spattering method, evaporation method or plating method is applied onmetals for forming electrode such as Ni, Ai, Cu, Al and the like to forma metal layer on the upper surface of the dry film left without grindingand on the inner face of each grooves.

After that, the dry film as well as the metal layer formed on thesurface of the dry film is removed, to obtain a board where the metallayer is fanned only on the inner face of the each groove.

Next, cover plate 24 is adhered to cover the each groove with anadhesive agent, and the board with the cover plate 24 is cut along thedirection perpendicular to the longitudinal direction of the groove,thus, a plurality of head chips 310 each having channel rows are formedat one time. In the case of a head for time-division drive as shown inFIGS. 4 a and 5 a-c, which has no air channel, each groove becomeschannel 28, and in the case of a head for independent drive as shown inFIGS. 7 a and 8 a-c, which has air channels, each groove becomes inkchannel 28 or ink channel 128. The metal layer in each groove becomeselectrode 29, and the part between the adjacent grooves configured withpiezoelectric materials 27 a and 27 b, which are different in thepolarized direction with each other by sandwiching the connectionportion, becomes partition wall 27. Width between the cutting linesdetermines the drive length (shown by L in FIG. 4 b) of channel 28 inhead chip 310 produced by the cutting, and this width is properlydetermined according to the drive length.

After that, after fixing head chip 310 and wiring board 102, patterningis conducted on the front face of head chip 310 and front edge face andsurface (the opposite face to the adhered face of head chip 310) ofwiring board 102. After that, aluminum is evaporated, and the dry filmis removed as well as the aluminum layer formed on the dry film, thus,each connection electrode 300 connected to electrode 29 in each channelis formed at a time.

The forming method of A1 layer is not restricted to evaporation, but anycommon thin layer forming method may be applied. Inkjet coating ofelectro-conductive paste may be applied. After forming the A1 layer, byremoving the dry film with solvent peeling, the A1 layer formed on thedry film is removed, and only connection electrode 300 is remained onthe front face of head chip 310 and on the front edge face and surfaceof wiring board 102.

In the head having air channels, on the rear face of head chip 310, aflow path restriction member 302 to prevent ink flow into air channel128 is adhered so as to entirely close the opening of each air channel.In cases of the head not having the air channel, this kind of flow pathrestriction member is not provided.

After that, flow path board 104 is fixed. Then, enclosing wall 103 isfixed to enclose the rear face of head chip 310 extending from wiringboard 102 through flow path board 104, and to form a common ink room 77.After that, flexible cable 6 is connected to each connecting electrode300 of wiring board 102.

Next, a plate of nozzle forming member 22 formed with nozzle 23 isadhered via adhesive agent on head chip 310.

As a material of nozzle forming member 22, other than synthetic resinssuch as polyimide resin, polyethylene terephthalate resin, liquidcrystal polymer, aromatic polyamide resin, polyethylene naphthalateresin, or polysulphone resin, a metal material such as stainless steelmay be used.

The two chips, where electrodes and common ink room are formed asdescribed above, are set position such that two nozzle rows are shiftedby a half pitch with each other, and wiring boards of the two chips areadhered with an adhesive so as to face with each other. Thus a headhaving two nozzle rows arranged in zigzag and having twofold resolutioncan be produced.

Next, an ink ejection operation will be described.

As shown in the present embodiment, in cases where the inkjet head isconfigured with the piezoelectric material to be deformed with the shearmode, a rectangular wave (to be described later) can be more effectivelyutilized to lower the drive voltage and to enable an efficient driveoperation.

Drive signal generator 100, or 101 includes a drive signal generationcircuit (not illustrated) which generates a series of drive signalsincluding at least one drive pulse for each one pixel period, and adrive pulse selection circuit (not illustrated) which selects andsupplies to each pressure chamber a drive pulse out of the drive signalssupplied from the drive signal generator in accordance with image dataof each pixel. And the drive signal generator supplies a drive signal todrive partition wall 27 as a pressure generation section.

Upon receiving the image data, a controller (not illustrated) controls amortar of conveyance rollers and allows the drive signal generator togenerate a drive pulse which includes at least on pulse and off pulse.Further the controller outputs, to the drive pulse selection circuit,information of drive pulse to be selected based on the image data. Thusthe drive pulse selection circuit selects a drive pulse based on theinformation and supplies to electrode 29 covering partition wall 27. Bythe above, an ink droplet can be ejected in one pixel period from nozzle23 of recording head 31.

Next, the drive signal and the ejecting operation will be described.

When the drive signal from drive signal generator 100, 101 is appliedonto electrode 29A, 29B, and 29C (electrode 29 for each channel), an inkdroplet is ejected from nozzle 23 according to the operation exemplifiedbelow. In FIGS. 5 a-5 c, nozzles are omitted, and in FIGS. 8 a-8 c,indicated are nozzles formed only on ink channels.

In said recording head 31, positive or negative pressure is applied tothe ink in ink channel 28 by the deformation of partition wall 27, thuspartition wall 27 constitutes a pressure generation section.

First Embodiment

Next, a time division drive system which being an example of the drivemethod relating to the present embodiment is explained

In the case of driving recording head 31, as shown in FIGS. 4 a-b and 5a-c, containing multiple pressure chambers 28 which are partitioned bypartition walls 27 each of which is at least partially made ofpiezoelectric materials, when one of ink channels 28 works to eject ink,the neighboring ink channels 28 are affected. To prevent this, themultiple ink channels 28 are usually grouped into two or more groups,each of the groups including pairs of ink channels sandwiching one ormore ink channels of the other group. These pressure chamber groups arecontrolled in sequence to eject ink in a time-division manner. Forexample, three cycle ejecting method is conducted where all the inkchannels 28 are grouped into three groups each containing every thirdchannels to eject ink in three phase.

The 3-cycle ejection operation will be further explained referring toFIGS. 6 a-6 c. In the example shown in FIGS. 6 a-6 c, the recording headcontains nine ink channels 28 (A1, B1, C1, A2, B2, C2, A3, B3, and C3).

At the time of ejection, voltages are applied to electrodes ofrespective ink channels 28 of group A (A1, A2, and A3), while theelectrodes of the pressure chambers of neighboring groups B and C aregrounded. By applying a drive signal including the first expansionpulse, contraction pulse, and the second expansion pulse to theelectrode of ink channels 28 of group A, a minute droplet of ink isejected from the nozzle connecting to the pressure chamber of A group.

After an elapse of certain time, each ink channel 28 of group B (B1, B2,and B3) and group C (C1, C2, and C3) are similarly operated in sequence.

FIG. 10 shows a drive signal to realize the drive method of theembodiment relating to the present invention.

In FIG. 10, the horizontal axis represents AL time, and the verticalaxis represents drive voltage.

Sign t1 represents the width of first expansion pulse.

Sign t2 represents the width of contraction pulse.

Sign t3 represents the width of second expansion pulse.

(1) In the state shown in FIG. 5 a of recording head 31, when electrode29A and 29C are grounded and electrode 29B is applied the firstexpansion pulse (positive voltage) of rectangular waveform, by rising ofthe first expansion pulse (P1), voltage of Von is applied to caused anelectric field perpendicular to the direction of polarization ofpiezoelectric materials 27 a and 27 b which constitute partition walls27B and 27C. This causes a shearing deformation in the jointed surfaceof partition walls of piezoelectric materials 27 a and 27 b.Consequently, as shown in FIG. 5 b partition walls 27B and 27C bothdeform outward to expand the volume of ink channel 28B and therebygenerate negative pressure to the ink in ink channel 28B to cause theink to flow in (Draw).

Wherein, AL (Acoustic Length) is ½ of the acoustic resonance cycleperiod of the ink channel. AL can be obtained as a pulse width whichmaximizes the ejection velocity of ink droplets when the pulse width ofrectangular pulses is varied with the rectangular pulse voltage keptconstant in measurement of the ejection velocities of ink droplets whichare ejected by applying rectangular pulses to partition wall 27 which isan electro-mechanical transducer. This value is determined depending onthe head structure and the ink density.

Wherein pulse is a rectangular wave having a constant wave heightvoltage, and when 0V is assumed 0%, and the wave height voltage isassumed 100%, “pulse width” is defined as the interval respectivelybetween the point of 10% voltage in the rise or fall from the voltage of0V and the point of 10% voltage in the fall or rise from thepulse-height voltage.

Further, “rectangular wave” is assumed to be a waveform both of whoserise and fall time periods between 10% and 90% of the drive voltage arewithin 1/10 of AL and preferably within 1/20.

(2) Since the pressure wave in ink channel 28B repeats reversals at each1 AL time period, when the voltage is returned to 0 volt (P2) after alapse of 1 AL from the application of the first P1, partition walls 27Band 27C returns from the expansion position to a neutral position asshown in FIG. 5 a to cause a high pressure on the ink in ink channel28B. Here, the first expansion pulse width t1 is preferably 1 AL.

Successively, a contraction pulse (negative voltage Voff) of rectangularwaveform is applied. Due to a falling edge of the contraction pulse(P3), partition walls 27B and 27C deform reversely with each other andthe volume of channel 28B decreases. Due to this contraction, furtherhigh pressure is applied to the ink in ink channel 28B and an ink columnis protruded from an opening of nozzle 23.

(3) After the elapse of t2 time period, when the voltage is returned to0 and the second expansion pulse (Von) is successively applied (P5), thevolume of channel 28B expands to cause a high negative pressure on theink in ink channel 28B. Thus, a meniscus is drew-in and the rear edge ofthe protruded ink column is drew back to make the ink column diametersmall and cut off an ink droplet.

After the elapse of t3 time period from P5, the voltage is returned to 0to make the state of FIG. 5 a, thus the pressure wave can be rapidlydecreased.

The head is driven by repetition of the above described series of drivepulses. Therefore, the faster the rate of pressure wave decreases thefaster the ink for next pixel can be ejected to enable the higher speedprinting, which being preferable.

Width of the first expansion pulse largely affects to an ejection powerof the ink droplet, and when this pulse width becomes 1 AL the inkejection power (ejection speed) is maximized. Further the contractionpulse is applied at the falling edge of the first expansion pulse (P2),namely after the elapse of 1 AL. Thus, by setting the width of the firstexpansion pulse to 1 AL, at the same time when the negative pressurewave generated at rising edge of the expansion pulse (P1) transfersthrough the ink channel and reverses to positive pressure, the positivepressure, which is generated with the contraction of ink channel causedby falling edge (P2) of the expansion pulse and falling edge (P3) of thecontraction pulse, is added, the most effective ejection power can beobtained with all of these effects. Therefore, advantage of highejection speed of the ink can be attained.

Further, by setting the width of contraction pulse to be 0.1-0.5 AL,small droplets can be formed. In cases of less than 0.1 AL, since thetime for the drive walls to respond is not sufficient, the droplet isvolume cannot be decreased. In cases where the contraction pulse widthexceeds 0.5 AL to become 0.6 AL, the droplet volume becomes rapidlylarge, which is not preferable.

Further, there are cases where the volume of the ink droplet is requiredto be adequately set according to conditions of resolution and gradationof the image. Further, the volume of the ink droplet is affected by atemperature of the recording head and the like. For example, in caseswhere the temperature of the recording head is low, a volume of theejected ink droplet becomes small and a recorded dot area becomes small.On the contrary, in cases where the temperature of the recording head ishigh, a volume of the ejected ink droplet becomes large and a recordeddot area becomes large. Namely even in cases where recording is executedwith the same image data and with the same drive pulse, if thetemperature of the recording head is unstable, the size of dots formedon the recording medium, and consequently the image density will beunstable, and uneven density of the image will be caused.

Due to the above, it is preferable to control the volume of ink dropletby varying the contraction pulse width in the range of 0.1 AL through0.5 AL.

The volume control of the ink droplet ejected from the recording headcan be executed by the control section such as the CPU controlling drivesignal generator 100 or 101, through modulation of the contraction pulsewidth in the drive pulse. Namely, as described below in case ofrequiring small ink droplet volume, the width of the contraction pulseis made small, and in case of requiring large ink droplet volume, thewidth of the contraction pulse is made large.

Thus, regardless the temperature of the recording head, for example, thevolume of the ink droplet can be controlled in a prescribed controlrange. Further, according to the conditions of resolution and gradationof the image, the volume of ink droplet can be increased or decreased.

From the points of droplet volume and upper limit of stable ejectionspeed, the width of second expansion pulse is preferably 0.2 through 0.6AL (not less than 0.2 AL and not more than 0.6 AL), and more preferablyis 0.4 through 0.6 AL. In case of less than 0.2 AL, the droplet volumeincreases, and is not preferable for ejecting a small droplet. In caseof exceeding 0.6 AL, the maximum stable ejection velocity of the dropletdecreases rapidly, and is not preferable.

Further, the width of the contraction pulse is preferably smaller thanthe width of the second expansion pulse.

Further, a sum of widths of the contraction pulse and the secondexpansion pulse is preferably not less than 0.3 AL and not more than 0.9AL.

Further, in the above embodiment the width of the first expansion pulseis set to be 1 AL, however it may be set to be not less than 0.7 AL andnot more than 1.3 AL. Beyond this range, ejection efficiency by thepressure wave decreases, and the drive voltage needs to be largelyincreased.

FIG. 9 shows an example of the drive signal described in PatentDocument 1. In this example, |Von|/|Voff|=1 is satisfied. Interval t1between the first expansion pulse and the first contraction pulse is 1AL, interval t2 between the first contraction pulse and the secondexpansion pulse is 0.5 AL, and pulse width t3 of the second expansionpulse is 0.5 AL. Wherein |Von| represents the absolute value of Von, and|Voff| represents the absolute value of Voff.

FIG. 10 shows the drive signal of the present invention. Here,|Von|/|Voff| is in the range of 1.3 through 10, where a drive voltage ofthe first expansion pulse is Von and a drive voltage of the contractionpulse is Voff. Interval t1 is 1 AL, interval t2 is 0.1 through 0.3 AL(not less than 0.1 AL and not more than 0.3 AL), and pulse width t3 is0.2 through 0.6 AL, thus one cycle period of the drive signal is madeshorter than the drive signal of FIG. 9.

The ratio of |Von|/|Voff| is required to be in the range of 1.3 through10 from the points of droplet volume and length of satellite, ispreferably 2 through 10, and is more preferably 3 through 10.

When droplets are ejected from the nozzle, the droplets fly in such astate that an ink column is extended in the rear direction from a maindroplet. The ink column at the rear position becomes satellites (smalldroplets) before arriving to a recording medium. Wherein the longer thesatellite length (distance from the main droplet to the rear mostsatellite), the more increased is volume of the satellites, which causesdistortion of the image.

Further, the drive voltage of the second expansion pulse is set to besame as the drive voltage Von of the first expansion pulse. This ispreferable in reducing the cost of drive signal generator 100 or 101 forgenerating the drive pulse by reducing the number of power supplyvoltages to reduce the circuit cost.

In the drive signal of the present embodiment, off-waveform correspondsto the contraction pulse and on-waveform corresponds to the first andthe second expansion pulses. Further, GND (ground potential) isselectable in the waveform. Herein, since the drive voltage of the firstexpansion pulse is set to be same as the drive voltage of the secondexpansion pulse, each of the on-waveform and the off-waveform can begenerated by merely digitally switching respective single power voltageof Von or Voff.

In the above embodiment, although an example of a drive signal whichapplies a single drive pulse in one pixel period to eject the inkdroplet, the other drive signal may be utilized, which applies pluraldrive pulses each causing to eject respective ink droplet in one pixelperiod.

FIG. 11 shows the drive signal which applies plural drive pulses eachcausing to eject respective ink droplet in one pixel period. As for eachdrive pulse in one pixel period, the drive pulse similar to that shownin FIG. 10 is used. The plural drive pulses are sequentially applied ina condition that a drive pulse halt period (ground potential period) isarranged between each of the plural pulses.

According to the present embodiment, in cases where N pieces of dropletsejected by N pieces of drive pulses (N is an integer larger than 2) areunited during a fright before landing or united after landing on therecording medium to form a single super drop UD and to form a dot of onepixel, smaller sub-drops than prior art can be stably ejected with highdrive frequency.

By the drive signal shown in FIG. 11, N pieces of ink droplets areejected in maximum, and printing of gradations from 0-level to N-levelcan be performed.

For example, in the case of N=3, each of zero drop (0-level gradation),one drop formed with sub-drop SD₁ ejected by the first drive pulse inone pixel period (1-level gradation), two drops of SD₁ and SD₂ ejectedby the second drive pulse in the one pixel period (2-level gradation),and three drops of SD₁, SD₂ and SD₃ ejected by the third (last) drivepulse in the one pixel period (3-level gradation) can be formed torealize the printing from 0-level gradation to 3-level gradation.

One example will be shown that when pulse width t1 of the firstexpansion pulse is 1 AL, pulse width t2 of the contraction pulse is 0.2AL, and pulse width t3 of the second expansion pulse is 0.45 AL, thehalt period of drive pulse t4 is 3.28 AL. The halt period of drive pulset4 is preferably 0.7 AL through 5.2 AL.

Next, three cycle ejection operation will be further described referringto FIG. 12. Based on the drive signal of FIG. 11, an example where Npieces of ink droplets (sub-drop) are ejected in one pixel period willbe described. A timing chart of the drive signals to be applied on theelectrode of pressure chamber of each group of A, B, and C is shown isFIG. 12.

A period to form a super drop by N pieces of sub-drops SD₁-SD_(N) isassumed to be one pixel period.

At the time of ejecting ink, firstly a series of drive pulse voltages isapplied to the electrode of each pressure chamber 28 in A group (A1, A2,and A3) for ejecting said SD₁-SD_(N), with grounding the electrodes ofadjoining pressure chambers of both sides, and ink droplets SD₁-SD_(N)are ejected.

Subsequently, each pressure chambers in B group (B1, B2, and B3) areoperated, and further subsequently each pressure chambers in C group(C1, C2, and C3) are operated similarly to the above.

Although the above described is the case for a solid image (full drivecase), in actual the number of droplets to be ejected among SD₁-SD_(N)is varied according to the print data of each pixel.

Further, a case is also possible where the plural drive pulses include aplurality of drive pulses, each having plural types of differentcontraction pulse width, and ink droplets of different volumes areejected by each drive pulse, and then the ejected plural ink dropletsare united before or after landing onto the recording medium to form asingle pixel. According to this case, gradation can be improved.

Second Embodiment

Next, an independent drive head, which is an example of drive methodrelating to an embodiment of the present invention, will be described.

In the case of driving recording head 31, as shown in FIGS. 4 a-b andFIGS. 5 a-c, containing multiple ink channels 28 which are partitionedby partition walls 27 each of which is at least partially made ofpiezoelectric materials, to prevent the influence on neighboring inkchannel 28 at the time of operating the partition wall of one of inkchannels 28 to eject ink, a channel row is formed where ink channel 28and air channel 128 are alternately arranged. Since the air channel 128exist between each ink channel 28, an ink channel 28 is not influencedby the operation of partition wall of the neighboring ink channel 28.Thus, as shown in FIG. 7 a, in an independent chive head, pressurechamber having an ink inlet and a nozzle is arranged at every twochannels in the channel row.

Therefore, since in said independent drive head each pressure chambercan be driven by concurrently applying the drive signals shown in FIG.10 or FIG. 11, the pressure chambers are not divided into A group, Bgroup or C group. Regarding the other conditions, the drive in thesecond embodiment can be similarly executed to the first embodiment.

EXAMPLE Example 1

Two set of shear mode type three-cycle drive head as shown in FIGS. 4a-b, and FIGS. 5 a-c(nozzle pitch: 180 dpi, number of nozzles: 512,nozzle diameter: 27 μm, AL: 5.3 μs) are prepared, and adhered such thateach nozzle row is shifted by ½ pitch with each other to form a zigzagarrangement. Since each is a head of 180 dpi, by shifting the eachnozzle row by ½ pitch, the adhered head can be used as a recording headwith 360 dpi which being a high recording density head having increasednumber of nozzles.

With supplying the ink to this head having two rows (nozzle pitch: 360dpi, number of nozzles: 1024), the drive signal described below isapplied to each channel of the head. Channels in the channel row aredivided to three groups, and the three-cycle drive is executed with theconditions described below.

Ink: Mixed organic solvent type {(viscosity: 10 mPa·s, surface tension:30 mN/m (measured at 25° C.));

<Drive Signal and Droplet Volume>

Drive frequency: 12.6 kHz;

Drive voltage ratio of expansion pulse and contraction pulse:|Von|/|Voff|=2;

t1 (first expansion pulse width)=1 AL;

t2 (contraction pulse width): varied as shown in FIG. 13 (varied in therange of 0.1 AL through 0.65 AL);

t3 (second expansion pulse width): varied as shown in FIG. 13 (0.3 AL,0.45 AL, or 0.6 AL); and

Drive voltage Von was 12.5-17.5V (ejection experiment is conducted byvarying the drive voltage in the range of 12.5 through 17.5V to measuredroplet velocities and droplet volumes).

The droplet volume is shown in FIG. 13.

The droplet volume is relatively small in the condition of contractionpulse width 0.1-0.5 AL.

In contrast, the droplet volume of a comparative example where the samehead and ink as described above are used and the head is driven with thesame drive frequency and with a similar drive signal as described in thePatent Document 1: (|Von|/|Voff|=1, t1=1 AL, t2=0.5 AL, t3=0.5 AL) was10.9 pl, which is indicated by a dashed line in FIG. 13.

The droplet volume in the case of varying the ratio of |Von|/|Voff| isshown in FIG. 15. In FIG. 15, in cases where |Von|/|Voff| is 1.3 ormore, the droplet volume becomes significantly small.

<Length of Satellite>

In the conditions where the contraction pulse width t2=0.3 AL, and thesecond expansion pulse width t3=0.45, the satellite length was evaluatedby varying |Von|/|Voff|, and the evaluation result is shown in Table 1.

TABLE 1 Length of Test No. |Von|/|Voff| satellite Remark 1 1.3 A Presentinvention 2 3 A Present invention 3 5 A Present invention 4 10 A Presentinvention 5 20 B Comparative example 6 50 C Comparative example In Table1, A, B, and C respectively represents the evaluation result as below,A: Main droplet and satellites landed on the recording medium at thesame position, and no distortion is observed in dot shape. Roughness inthe image is not observed at all. B: Main droplet and satellite landedsomewhat separately on the recording medium, and dot shape is slightlydistorted. Roughness in the image is somewhat observed. C: Main dropletand satellite landed separately on the recording medium, pixel isdisturbed and the dot shape is distorted. Remarkable roughness in theimage is observed.

As shown in Table 1, the dot shape of the image is not distorted androughness of the image is not observed in cases where |Von|/|Voff| is inthe range of 1.3 through 10.

<Drive Signal and Pressure Wave>

In the state of |Von|/|Voff|=2, by the same conditions as the above,except that the contraction pulse width and the second expansion pulsewidth are varied as described below, simulation of decaying state of thepressure wave after the head having been driven is executed. The resultis shown in FIGS. 16-18. In FIGS. 14-16, the vertical axis represents arelative value of the pressure.

FIG. 16 (Present Invention), contraction pulse width: 0.2 AL, secondexpansion pulse width: 0.5 AL,

FIG. 17 (Present Invention), contraction pulse width: 0.2 AL, secondexpansion pulse width: 0.1 AL,

FIG. 18 (Present Invention), contraction pulse width: 0.2 AL, secondexpansion pulse width: 0.8 AL,

From these figures, it is understood that FIG. 16, which satisfies thecondition of the second expansion pulse width being 0.2 AL through 0.6AL, represents faster decay of the pressure wave than FIG. 17 or 18which do not satisfy the above condition.

<Maximum Stable Ejection Velocity>

In each drive signal applying conditions, while increasing the flyingvelocity of ink droplet by raising the drive voltage, the flyingconditions are observed. Upper limit of flying velocity, that does notcause unstableness in ejection due to air being took in the pressurechamber, is defined as maximum stable ejection velocity.

In the state of |Von|/|Voff |=2, by the same conditions as the above,except that the contraction pulse width is varied in the range of 0.1 ALthrough 0.3 AL, and the second expansion pulse is varied in the range of0.2 AL through 0.7 AL, ejection experiments are executed and the maximumstable ejection velocity (upper limit of stable flying velocity of thedroplet) is illustrated in FIG. 14. From FIG. 14, it is understood thatin conditions where the second expansion pulse width is in the range of0.2 AL through 0.6 AL, the maximum stable ejection velocity is kept highcompared to the case of 0.7 AL; in conditions where the second expansionpulse width is in the range of 0.2 AL through 0.5 AL, the maximum stableejection velocity is kept further in high level, and in conditions wherethe second expansion pulse width is in the range of 0.2 AL through 0.4AL, the maximum stable ejection velocity is kept in highest level.

In contrast, by the same conditions as the above, except that|Von|/|Voff|=1, contraction pulse width=0.5 AL, and second expansionpulse width=0.5 AL (these conditions correspond to the drive signaldescribed in Patent Document 1) the maximum stable ejection velocity ismeasured and illustrated in FIG. 14, which is lower than the maximumstable ejection velocity of the present invention. This showseffectiveness of the present invention.

Relationships between drive frequency and maximum stable ejectionvelocity are illustrated in FIG. 19 in cases where ejections areexecuted in the drive conditions of the present invention(|Von|/|Voff|is 1.3 through 10, first expansion pulse width=1 AL, contraction pulsewidth=0.27 AL, and second expansion pulse width=0.45 AL), and inconditions of the drive signal of Patent Document 1 (|Von|/|Voff|=1,first expansion pulse width=1 AL, contraction pulse width=0.5 AL, andsecond expansion pulse width=0.5 AL). From FIG. 19 it is understood thataccording to the present invention, the maximum stable ejection velocityis maintained high through the wide range of drive frequency. Ingeneral, in cases of increasing the ink ejection velocity, the ejectionbecomes unstable due to that the air being took in the pressure chamberand the like. The maximum stable ejection velocity is referred as theupper limit of velocity with which the ink droplet is stably ejected.From the above, it will be understood that by utilizing the drive methodof the present invention, high frequency drive is enabled andimprovement of printing speed will be realized.

Example 2

By utilizing a similar inkjet head as that of EXAMPLE 1 except for theconditions of nozzle diameter=20 μm and AL=3.0 μs, and applying thedrive signal based on that shown in FIG. 10, the same ink as used inEXAMPLE 1 is ejected.

The drive signal:

t1=1 AL,

t2=varied in the range of 0.1 through 0.6 AL,

t3=0.45 AL,

|Von|/|Voff|=2,

Drive frequency=22.2 kHz.

Drive voltage was 18-21V.

Relationship between the contraction pulse width and the liquid dropletvolume is illustrated in FIG. 20.

Shown in FIG. 20 is that in cases where the contraction pulse width isin the range of 0.1 through 0.5 AL, the droplet volume remains small,and in cases where the pulse width exceeds that range, the dropletvolume rapidly increases.

Example 3

By utilizing a similar inkjet head as that of EXAMPLE 1 except for theconditions of nozzle diameter=30 μm, AL=4.5 μs, and the head being anindependent driven shear mode type, and by applying the drive signalbased on that shown in FIG. 10, the ink described below is ejected.

The drive signal:

t1=1 AL,

t2=varied in the range of 0.1 through 0.8 AL,

t3=0.45 AL,

|Von|/|Voff|=2,

Drive frequency=20 kHz.

Ink:

Mixed liquid of water and organic solvent,

Viscosity=5.7 mPa·s

Surface tension: 41 mN/m,

Drive voltage was 11-22V.

Relationship between the contraction pulse width and the liquid dropletvolume is illustrated in FIG. 21.

Shown in FIG. 21 is that in cases where the contraction pulse width isin the range of 0.1 through 0.5 AL, the droplet volume remains small,and in cases where the pulse width exceeds that range, the dropletvolume rapidly increases.

Example 4

By utilizing a similar inkjet head as that of EXAMPLE 1 except for theconditions of nozzle diameter=20 μm, AL=3.6 μs, and by applying thedrive signal based on that shown in FIG. 10, the ink described below isejected.

The drive signal:

t1=1 AL,

t2=0.27 AL,

t3=0.45 AL,

|Von|/|Voff|=2,

Drive frequency=22.2 kHz.

Ink:

Ink composed of silver nanoparticles dispersed in organic solvent,

Viscosity: 8.9 mPa·s,

Surface tension: 26 mN/m,

Drive voltage was 16.4 V.

The liquid volume in the case of applying the above drive signal was 1.4pl, in contrast to the liquid volume having been 2.2 pl in the case ofapplying drive signal of Patent Document 1. Since it is enabled to ejectsuch a fine particles, the present invention is particularly effectivein cases of utilizing on a circuit board that requires the drawing withfine lines.

1. An inkjet recording apparatus comprising: a recording head having anozzle to eject an ink droplet, a pressure chamber connected to thenozzle, and a pressure generation section to vary a volume of thepressure chamber; and a drive signal generator which generates a drivesignal for applying at least one drive pulse within one pixel period toeject the ink droplet, wherein the inkjet recording apparatus isconfigured to eject the ink droplet from the nozzle by applying thedrive signal to activate the pressure generation section, wherein thedrive signal generator is configured to generate the drive signal withinone pixel period in chronological order including a first expansionpulse to expand the volume of the pressure chamber, a contraction pulseto contract the volume of the pressure chamber, and a second expansionpulse to expand the volume of the pressure chamber again, and wherein apulse width of the first expansion pulse is greater than 0.8 AL and lessthan 1.3 AL, a pulse width of the contraction pulse is not less than 0.1AL and not more than 0.5 AL, where AL represents a half of an acousticresonance period of the pressure chamber, and |Von|/|Voff| is not lessthan 1.3 and not more than 10, where Von represents a drive voltage ofthe first expansion pulse, and Voff represents a drive voltage of thecontraction pulse.
 2. The inkjet recording apparatus of claim 1, whereina pulse width of the second expansion pulse is not less than 0.2 AL andnot more than 0.6 AL.
 3. The inkjet recording apparatus of claim 1,wherein a sum of a pulse width of the contraction pulse and a pulsewidth of the second expansion pulse is not less than 0.3 AL and not morethan 0.9 AL.
 4. The inkjet recording apparatus of claim 1, wherein apulse width of the first expansion pulse is 1 AL.
 5. The inkjetrecording apparatus of claim 1, wherein a pulse width of the contractionpulse is less than a pulse width of the second expansion pulse.
 6. Theinkjet recording apparatus of claim 1, wherein the apparatus varies apulse width of the contraction pulse within a range of 0.1 AL through0.5 AL to control a volume of the ink droplet.
 7. The inkjet recordingapparatus described claim 1, wherein the drive signal generator isconfigured to generate the drive signal for applying a plurality ofdrive pulses within one pixel period, each of the plurality of drivepulses including the first expansion pulse, the contraction pulse andthe second expansion pulse, in such a manner that a plurality of inkdroplets each ejected by each of the plurality of drive pulses areunited before or after landing on recording medium to form a singlepixel.
 8. The inkjet recording apparatus of claim 7, wherein theplurality of drive pulses include plural types of drive pulsesrespectively having contraction pulse widths different with each otherwithin a range from 0.1 AL through 0.5 AL, and the respective pluraltypes of drive pulses cause to eject plural types of ink droplets havingdifferent volumes with each other.
 9. The inkjet recording apparatus ofclaim 1, wherein a drive voltage of the second expansion pulse is equalto a drive voltage Von of the first expansion pulse.